The Aquitaine Basin

The Aquitaine Basin is located in the southwest of France, between the Gironde Arch in the north and the Pyrenean Mountain Chain in the south. It is a triangular-shaped domain, extending over 35 000 km2. From north to south, six main geological provinces can be identified: (1) the Medoc Platform located south of the Gironde Arch; (2) the Parentis sub-basin; (3) the Landes Saddle; (4) the North Aquitaine Platform; (5) the foreland of the Pyrenees (also known as the Adour, Arzacq and Comminges sub-basins); and (6) the Pyrenean fold-and-thrust belt. Only the Parentis sub-basin, the foreland of the Pyrenean Chain and a minor part of the fold-and-thrust belt itself are proven hydrocarbon provinces.

The Aquitaine Basin, in turn, is subdivided into four sub-basins – the Parentis, Adour–Arzacq, Tarbes and Comminges areas. The lozenge shape of these depocentres is related to the Hercynian tectonic framework of the Palaeozoic basement, reactivated during Early Cretaceous rifting. This rift phase aborted at the end of the Albian (prior to the development of an oceanic crust) in response to the beginning of the subduction of the Iberian plate under the European plate. During the Upper Cretaceous, continued subduction led to the creation of northwards-migrating flexural basins. In the Eocene, a paroxysmal phase of compression was responsible for the uplift of the Pyrenean Mountain Chain and for the thin-skinned deformation of the foreland basin. The resulting structuration is limited to the south by the internal core of the chain and to the north by the leading edge of the fold-and-thrust belt, where the Lacq and Meillon gas fields are located.

Four main petroleum provinces have been exploited since the Second World War: (1) the oil-prone Parentis sub-basin and (2) salt ridges surrounding the Arzacq and Tarbes sub-basins; and (3) the gas-prone southern Arzacq sub-basin (including the external Pyrenean fold-and-thrust belt and the proximal foreland sub-basin) and (4) Comminges sub-basin. Two major distinct vertically drained petroleum systems (PS) can be identified: the Upper Kimmeridgian–Barremian is the main PS, based on Type II shaly-carbonate source rocks; and the Lias PS based on Type II–III source rocks. The latter is restricted to the Comminges and Tarbes sub-basins. Reservoirs consist of fractured and diagenetically modified carbonates, and clastics. Shaly-marly deposits of regional extent associated with transgressive systems provide the main seals.

Formation of petroleum traps results from a complex polyphase geodynamic evolution. They are developed mainly along Palaeozoic inherited palaeostructures. Oil and gas migration took place from Albian times up to the present, depending on the respective structural position and histories of the traps. These petroleum provinces of the Aquitaine Basin have been exploited since 1939 and a total of 11.5×1012 SCF of gas and 470×106 BBL oil recoverable reserves have been proven to date (the total amount is in the order of 2.5×109 BOE).